OBJECTIVES: A. To evaluate hydrophylic polyurethane prepolymers as possible agents to be used for embolizing tumors through percutaneous catheters. B. To determine tissue toxicity of such plastics if they prove technically suitable as transcatheter embolizing agents. METHODS EMPLOYED: A variety of techniques for reducing the viscosity and decreasing the expansion of hydrophylic polyurethane prepolymers were investigated. Dimethyl sulfoxide was originally used as a diluent but caused hemorrhagic pneumonias when excreted through the lungs in animals. In addition, the expansion of these polymers due to the generation of CO2 at the time of polymerization made it difficult to prevent proximal propagation of the embolus and spill over into major vessels. These initial technical evaluations were accomplished in a series of dogs, cats and monkeys. At our request, Dr. Louis Wood at W.R. Grace and Co. developed a new polyurethane prepolymer that generated less CO2 during polymerization and had reduced viscosity. Transcatheter embolization of a number of spleens, livers and kidneys in a o series of cats and monkeys proved the material to be an effective embolizing agent. Acute toxicity studies in a serie of six lambs were performed. Right kidneys were embolized with Hypol and serial chemical determinations over three months were obtained. The embolized kidneys were examined histologically at the time of autopsy. No significant abnormalities of renal or hepatic function were apparent from the serial chemical determinations. At autopsy, the embolized kidneys were grossly infarcted but in many instances dissolution of arterial walls and transmural migration of the polymer had occurred suggesting significant delayed tissue toxicity.